Buffer and amplifier circuits include emitter follower circuits of the type illustrated in FIG. 1. The emitter follower is typically characterized by a high input impedance and a low output impedance. Therefore, it is useful as an isolation or buffer amplifier (e.g., a unity gain buffer) for connecting a high impedance source to a low impedance load.
With further reference to FIG. 1, when control terminal V+ is pulled sufficiently high, transistor Q1 is biased on to provide a corresponding voltage at positive output terminal Vout+. When control terminal V+ is pulled low, transistor Q1 is biased off and transistor Q3 draws current from a load (not shown) via the positive output terminal Vout+. Similarly, when control terminal V− is pulled high, transistor Q2 is biased on to provide a corresponding voltage at negative output terminal Vout−. When control terminal V− is pulled low, transistor Q2 is biased off and transistor Q4 draws a current from the load via the negative output terminal Vout−.
One disadvantage of the circuit shown in FIG. 1 is that it consumes excessive power. Specifically, transistors Q3 and Q4 are biased on by the current source 102 and transistor Q5. Therefore, when the control terminal V+ is pulled high, transistor Q1 provides the current drawn by transistor Q3 in addition to providing current to the load via positive output terminal Vout+. Likewise, when the control terminal V− is pulled high, transistor Q2 provides the current drawn by transistor Q4 in addition to providing current to the load via the negative output terminal Vout−. Thus, the current drawn by transistors Q3 and Q4 when transistors Q1 and Q2, respectively, are on is wasted.
For high frequency applications involving capacitive (including partly capacitive) loads, the overall impedance of the capacitive load is typically low. Therefore, the transconductance of the transistors must be increased in order to drive the low impedance. Additionally, the bias current of the circuit shown in FIG. 1 must be sufficiently large such that the output stage is not slew limited. As frequency is increased, emitter followers of the type shown in FIG. 1 require additional current in order to drive low impedance loads.
FIG. 2 illustrates a known common-emitter differential amplifier. When control terminal V+ is pulled low, transistor Q6 is biased off such that current is provided from voltage supply Vcc and through a resistor R1 to a load via negative output terminal Vout−. Conversely, when control terminal V− is pulled low, transistor Q7 is biased off such that current is provided from the voltage supply Vcc and through a resistor R2 to the load via positive output terminal Vout+.
In the case of capacitive loads, voltage supply Vcc is essentially coupled to an RC circuit. The load represents the capacitance C of the RC circuit. Thus, as the value R of resistors R1 and R2 is increased, the bandwidth of the differential amplifier decreases. Specifically, the bandwidth of the amplifier is given by:       f    BW    =      1          2      ⁢              π        ⁢        RC            
For high frequency applications, it is often desirable to have fBW>>fopt, where fopt is the operating frequency. If R is too small, however, excessive current will be conducted through transistors Q6 and Q7. This is because the amplitude of the output is given by ibiasR. Assuming transistors Q6 and Q7 are fully switched such that the current is steered to any one side during operation, a relatively large current is required to have sufficient amplitude during high frequency operation.
Referring now to FIG. 3, a class B push-pull amplifier is shown. When control terminal Vin is pulled high, transistor Q8 is biased on and transistor Q9 is biased off, resulting in transistor Q8 “pushing” current to a load via output terminal Vout. Conversely, when control terminal Vin is pulled low, transistor Q8 is biased off and transistor Q9 is biased on, resulting in transistor Q9 “pulling” current from the load via the output terminal Vout.
The buffer of FIG. 3 is generally efficient in terms of current consumption as compared to the buffer/amplifiers of FIGS. 1 and 2. However, transistor Q9 is typically a PNP bipolar junction transistor or a p-type MOS transistor. Because PNP bipolar junction transistors and p-type MOS transistors have less transconductance than their n-type counterparts, the buffer of FIG. 3 may either significantly load a previous stage to which the buffer is coupled, or the p-type transistor Q9 may be so weak that output waveforms are distorted. Further, p-type devices typically have low gm values and therefore are generally not well suited for high frequency applications.